CaptDon's TDS420 Oscilloscope Repair (w/Update)

[Smoky]

Many thanks to CaptDon for sending me his disabled TDS420 oscilloscope for the cost of shipping!

Not only that, he stuffed the TDS420 User's Manual, Programmer Manual, and the Reference Guide in the box too 

Now CaptDon's scope suffers from a bad power supply, but the fortunate thing is, I have a TDS420 parts machine with a bad Acquisition PC board but a newly restored power supply.

I'm hoping to make a working scope by combining parts.

The serial numbers on the scopes are B021343 and B021008 and both have the "13" option stamped on the tag.

Both Acquisition PC boards are stamped 671-1679-10 too!

Let the fun begin

[Smoky]

I did plan ahead for this moment. I stripped my chassis and cleaned the CRT assembly last Fall.

I started the assembly process by re-capping and installing the backplane board:





The power supply was cleaned, re-capped, new TVS installed, all solder joints re-flowed, and bench tested:





The make-or-break though is CaptDon's Acquisition board. I re-capped it tonight. CaptDon said that the scope passed SPC before the power supply went south. What I found was every original electrolytic capacitor was wet but there was minimal damage. I did find and repair one SMD ceramic capacitor near a leaking electrolytic capacitor with no continuity off of its soldering pad. We'll cross our fingers! I could not find any other lost connections:





I like using leaded electrolytic capacitors since they are easy to install and because I have a bunch of them!





The battery on the DSP board measured over 3 volts, so it was left alone. The CPU board battery was hovering around 2.6 volts, so it was replaced:





The Display board is part number 671-1784-02:





I know by now to go straight to inspecting and cleaning boards soon after pulling the scope from the shipping box. This is the membrane behind the front panel board. The leaking electrolyte makes its way through the holes from the scale knobs and gets trapped in the sandwich. It attacks the carbon and copper traces from behind. If your scale knob skips a step, this is where to look:





Even though CaptDon's Front Panel board had all of its original capacitors leaking, the board itself is in excellent condition. I'll be saving it for another project since I already have another FP board re-capped and ready to go:







I also cleaned and re-capped the RS-232 board:

[CaptDon]

I recapped the 644A CPU using all radial lead caps.
I recapped the 644A Aq board with SMD ceramic
caps. The original 644A Aq board had a bad memory
control mux chip which was unobtainium indeed so
I bought a 644A Aq board from Ebay and cleaned and
recapped it instead. The 644A now works 100%. My
644B did not have capacitor issues at all. There is a
world of difference between the 644A and 644B Aq
boards and the 644A does a 1,2,5,10 sequence in gain
or time settings and the 644B is different, something
like 1,2,4,10. Personally I prefer the 1,2,5,10 as it is
more like the analog scopes we grew up with and I am
used to that order of magnitude steps. I am only sad
that our Cal lab threw out our TDS540A saying they
could not get it to pass SPC. Idiots didn't return it to
me or send it for repairs, just threw in the metal scrap.

[Smoky]

Yep, I know what you mean. On a previous TDS420 repair project, the vertical scale knob would skip all of the "2's"

It turned out to be the leaking electrolyte of the capacitors on the front panel board leached through the holes to the back side and in between the rubber membrane. It severed the connection between the carbon pad at the start of the copper trace. A fine wire and some solder fixed the issue:





Btw, I'm almost ready to fire up the newly-rebuilt TDS420. A mix of parts were used from both scopes. I'll post some pictures of any good or bad errors that pop up 

[Smoky]

I love it!





And she's running right at 1.25amps at 120VAC:





The Signal Path Compensation test is coming next. I'm going to let the scope warm up for a half-hour or so then shut it down for a fresh re-start.

I have it out of its case for now. The case will go on for the SPC test:





So what really matters is, will it pass SPC?

...it sure does 





But here's the icing on the cake. CaptDon sent me his TDS420 and it was full of General Electric stickers on it. I thought it would be too much work to get them off     ...not!

And not only that, I noticed the case was coated differently than I had ever seen on a Tektronix oscilloscope. It has a very heavy texture with the correct color. All of the stickers came off with ease using Goo-B-Gone without leaving a hint of a mark or shade. So I took the handle off and put the case in the tub and scrubbed it good. Holy-crap, it's like new! It's Rhino-bedliner tough and in perfect shape.

Thank you CaptDon!

[CaptDon]

Very nice!! That makes two that were headed to the dumpster
now repaired, the TDS420 and the TDS644A that Norway Labs
said could be repaired for $1195 plus shipping both ways, which
I repaired for a total investment of $170. And there is also the
TDS644B that I actually pulled from the dumpster sitting in three
inches of water and repaired for about $10. I am thinking of selling
the TDS644A and donating any profit to the lab to buy hand tools.
Funny thing about industry, they often pay to have scrap hauled
away (all those dangerous CRT's and PCB's and stuff, don't get
me started) but an industrious person could live like a king fixing,
cleaning and re-purposing these industrial waste streams. We have
books in the cabinet for some outrageous Tektronix, HP and Wavetek
gear, and sadly the gear itself is gone!!! I bet we have 4 AM503
Current Probe Amplifiers each in a TM501 Power Housing but no
probes to be found. All of that stuff is currently on a shelf marked
scrap also!!! I did repair and calibrate one of the units and I have
the ferrite jaw 20A probe as well as the 70A gun stored with it
although I am the only one who ever uses it. Glad there are still
some of us old school hams and techies who love bringing this
old gear back to life and actually using it!!

[oventech]

Smoky great work on bringing the scopes back. Capt Don, i would be pissed too if a scope sent for cal was just tossed. I'd try to get the cost of the same scope out of them. Unless somebody higher up ok'ed the toss. Like the old Teks myself. Picked up 2 Tek 745's for 40.